1. Field of the Invention
This invention relates generally to an improved engine retarding method and apparatus of the compression release type. More particularly, the invention relates to a compression release retarding system for a four-cycle internal combustion engine which provides one compression release event and one bleeder event or two compression release events during each two revolutions of the engine crankshaft while utilizing only one intake valve opening event and at least partially disabling the normal exhaust valve opening event.
2. Prior Art
The problem of providing adequate and reliable braking for vehicles, particularly large tractor trailer vehicles, is well known. When such vehicles are operating at normal highway speeds they possess a very large momentum, and this may be increased substantially when the vehicle is required to negotiate a long decline. While the normal drum or disc type wheel brakes are capable of absorbing a large amount of energy over a short period of time, the absorbed energy is transformed into heat which rapidly raises the temperature of the braking mechanism to a level which may render ineffective the friction surfaces and other parts of the mechanism. As repeated use of the wheel brakes under these conditions is impracticable, resort has been made to auxiliary retarding devices.
Such auxiliary devices include hydraulic or electrodynamic retarding systems wherein the kinetic energy of the vehicle is transformed by fluid friction or magnetic eddy currents into heat which may be dissipated through appropriate heat exchangers. Other auxiliary systems include exhaust brakes which restrict the flow of air through the exhaust system and compression release retarder mechanisms wherein the energy required to compress the intake air during the compression stroke of a four cycle engine is dissipated by opening the exhaust valve near the end of the compression stroke so that the compressed air is exhausted during the expansion stroke of the engine. With respect to the engine compression release retarder, a portion of the kinetic energy of the vehicle is dissipated through the engine cooling system while another portion of the kinetic energy is dissipated through the engine exhaust system.
A principal advantage of the engine compression release retarder and the exhaust brake over the hydraulic and electrodynamic retarders is that both of the latter retarders require dynamos or turbine equipment which may be bulky and expensive in comparison with the mechanism required for the usual exhaust brake or engine compression release retarder. A typical engine compression release retarder is shown in the Cummins U.S. Pat. No. 3,220,392 while an exhaust brake is disclosed in Benson U.S. Pat. No. 4,054,156. A form of retarder that incorporates certain of the characteristics of the compression release retarder with those of the exhaust brake is known as the bleeder brake. In this mechanism, the exhaust or intake valves (or both) are maintained in a partially open position during the braking mode so that the engine consumes energy during pumping of the air through the partially open valves. Bleeder brakes are disclosed in the Siegler U.S. Pat. No. 3,547,087 and Jonsson U.S. Pat. No. 3,367,312. Other forms of compression release retarders are disclosed in Cartledge U.S. Pat. No. 3,809,033, Pelizzoni et al. U.S. Pat. No. 3,786,792 and Dreisin U.S. Pat. No. 3,859,970.
Since the advent of the Cummins Pat. No. 3,220,392 improvements have been made in various aspects of its operation while maintaining the same mode of operation, i.e., one compression release event for every two crankshaft revolutions. Such improvements include: a mechanism to prevent excess motion of the slave piston (Laas U.S. Pat. No. 3,405,699); a mechanism to prevent excess pushtube loading (Sickler U.S. Pat. No. 4,271,796); a mechanism to advance the opening of the exhaust valve during retarder operation (Custer U.S. Pat. No. 4,398,510; Price et al. U.S. Pat. No. 4,485,780); a mechanism to open only one of the exhaust valves during retarding (Jakuba et al. U.S. Pat. No. 4,473,047); and a mechanism to close the exhaust valve promptly after the compression release event (Cavanagh U.S. Pat. No. 4,399,787).
More recently, and in response to increased fuel costs and more stringent requirements with respect to air pollution, engine operating speeds have been decreased and the engine tuning specifications have been modified both of which adversely affect the performance of the engine retarder. In application Ser. No. 728,947, now U.S. Pat. No. 4,572,114 assigned to the assignee of the present application, a method and apparatus are disclosed by which two compression release events are produced during each two revolutions of the crankshaft for each engine cylinder. In accordance with this method, both the exhaust and intake valves are disabled from opening at the times required for the powering mode of engine operation. Means are provided to open the exhaust valve close to each top dead center (TDC) position of the piston and additional means are provided to open the intake valves during the ensuing expansion stroke as the piston moves toward the bottom dead center (BDC) position thereby providing an intake valve event corresponding to each compression release event. By providing two compression release events for each cylinder during every two revolutions of the crankshaft, the retarding horsepower developed by the engine can be increased substantially.